In recent years, the lithium secondary battery is widely used as the power supply of a personal computer, a portable device, and the like because it has a high power density. Furthermore, the lithium secondary battery has been studied for applications to the power supplies for environmentally-friendly electric vehicles and hybrid vehicles, and also applications to stationary power supplies and the like for absorbing the output fluctuation due to natural phenomena in combination with renewable energy power generation, such as photovoltaic power generation, or wind power generation. In the field of such large-sized lithium batteries, inexpensiveness and long-life as well as high-performance are required.
The examples of the positive electrode active material of the lithium secondary battery include LiCoO2, LiFePO4, and LiMn2O4. Although LiCoO2 is most promising in terms of the battery performance, cobalt as the raw material is expensive and thus it is difficult to reduce cost. Moreover, if the lithium secondary battery is held in high voltage state, Co will dissolve from the positive electrode active material and the battery life will decrease significantly. For LiFePO4, the raw material cost is inexpensive because iron is used but its manufacturing cost is high. Furthermore, in terms of the battery performance, LiFePO4 has a problem that the power density is low because the electron conductivity is low or the true density is low. On the other hand, LiMn2O4 is advantageous in terms of cost because the deposit of manganese as the raw material is 60 or more times as compared with that of cobalt, and furthermore the electron conductivity and the true density are approximately equal to those of LiCoO2.
However, LiMn2O4 has a problem that when the temperature thereof is increased, Mn will dissolve from the positive electrode active material, resulting in a decrease in the battery life. For such problems, for example, Patent Document 1 proposes that a part of manganese in the surface of the active material particle is substituted with a transition metal to form a surface layer, thereby suppressing the dissolution of manganese and achieving a long battery life. Although the thickness of the substitution layer in this method is not clear, judging from the description of claim 6 and the like the thickness of this substitution layer is estimated as on the order of 1 to 2 μm.
Moreover, Patent Document 2 proposes that a volume change of the positive electrode active material associated with charge and discharge is suppressed with a surface layer having a high fracture toughness value, thereby improving the cycle life.    [Patent Document 1] JP-A-2000-030709    [Patent Document 2] J-A-2003-178759